Additive manufacturing system and a method of additive manufacturing

ABSTRACT

An additive manufacturing system has a light permeable base, a build carrier for holding a workpiece and a light source which is arranged to emit light through the light permeable base. The light permeable base and the build carrier are positionable relative to each other in a build dimension in which the workpiece is built up. The system further has a resin vat in which the light permeable base forms a wall portion thereof. The system further comprises a plurality of resin supplies for supplying different light hardenable resins in direct contact with each other in said vat. The system facilitates the rapid manufacturing of a dental restoration having a color gradation.

FIELD OF THE INVENTION

The invention relates to an additive manufacturing system, and inparticular to an additive manufacturing system which has a plurality ofresin supplies for supplying different light hardenable resins in directcontact with each other on a light permeable base for building up aworkpiece from the different light hardenable resins. The inventionfurther relates to a method of additive manufacturing a workpiece.

BACKGROUND ART

In the field of dentistry, dental restorations, for example replacementteeth, are more and more manufactured in automated processes, whichtypically include the use of computer-aided design (CAD) techniques andmanufacturing by Computer Numerical Controlled (CNC) machines. Further,so-called build-up processes have been proposed for making of dentalrestorations. Such a build-up process typically allows building up anindividual dental restoration in substantially its desired individualshape, generally by subsequently adding material to create that shapeinstead of providing an oversized standardized blank from which materialis removed in a subsequent process.

While build-up processes are meanwhile widely used in the industry forrapid prototyping, the manufacturing of final products in many areas isstill challenging. For making dental restorations it is generallyrequired to use materials that are compatible for use in a human body.Further, a dental restoration manufactured by a build-up process mustfulfill requirements to mechanical stability as well as expectationsabout aesthetics, concerning for example color shading and translucency.

Some rapid prototyping systems are based on stereolithography.Stereolithography generally uses light (typically a UV laser) forhardening a light hardenable or photopolymer resin. Data based oncomputer aided design and/or computer aided manufacturing (CAD/CAM) areused to project a light pattern on a layer of light hardenable resin.The photosensitive resin typically solidifies in consequence of theexposure of the light so that a layer of solidified resin according tothe pattern is formed. A desired three-dimensional object is created byconsecutively adding layers. Thereby the pattern is controlled accordingto the desired outer shape of the three-dimensional object. Typicalstereolithography systems are configured for operating with a singleresin.

Although existing processes for making dental restorations areadvantageous in different respects there is a general desire to providea process for making individual or customized dental restorations at ahigh degree of automation, maximized quality and minimized costs.

SUMMARY OF THE INVENTION

The invention relates to an additive manufacturing system. The system ispreferably particularly configured for building up a dental restoration.The system comprises a resin vat comprising or forming a light permeablebase, a build carrier for holding a workpiece built up by the system anda light source which is arranged to emit light through the lightpermeable base toward a region between the light permeable base and thebuild carrier. The light permeable base and the build carrier arepositionable relative to each other in a build dimension in which theworkpiece is built up. The system further comprises at least one resinsupply for supplying different light hardenable resins in direct contactwith each other in said resin vat.

The invention is advantageous in that it allows building up a dentalrestoration in its final material structure and at a high level ofoptical aesthetics. In particular, the invention can be used for makinga dental restoration which does not need any sintering step. Further,the invention is advantageous in that it allows the making of dentalrestorations in tooth colors, including a tooth color gradation, whichresemble a natural tooth. Therefore the system and method of theinvention can be used in the dental industry as well as in a dentistspractice (“chairside”). The invention is also advantageous in that itallows for making a dental restoration rapidly, for example during or inparallel to the treatment of a patient.

Further the light permeable base preferably forms a build surface andthe build carrier forms a retention surface. The retention surfaceserves for retaining the workpiece to the build carrier. Thus, theworkpiece can be moved and positioned by moving and positioning thebuild carrier. The build surface and the build carrier preferably faceeach other. In particular, the build surface and the build carrier arearranged opposite of each other.

In a further embodiment the system comprises a plurality of resinsupplies for supplying different light hardenable resins. Each of theplurality of the resin supplies may be provided for supplying adifferent light hardenable resin.

In one embodiment the system is configured such that the system cancontrol the at least one resin supply for adding new light hardenableresin in direct contact with light hardenable resin present in the vatduring building up the workpiece.

In an embodiment the system is set up to build up a workpiece byperforming the following steps:

-   -   (a) using the at least one resin supply to provide a portion of        a light hardenable resin in the resin vat;    -   (b) positioning the build carrier and the light permeable base        at a predetermined distance relative to each other;    -   (c) emitting light by the light source toward the region between        the light permeable base and the build carrier for hardening the        hardenable resin in said region; and    -   (d) successively repeating steps (a) to (c),        whereby during building up the workpiece step (a) is performed        at least a first time for supplying a first light hardenable        resin in the resin vat and a second time for supplying a        different second light hardenable resin in the resin vat. This        means that during building up the workpiece at least two        different light hardenable resins are supplied in the vat.

The system may be set up to build up a workpiece by performing thefollowing steps:

-   -   (a) optionally using the at least one resin supply to provide a        portion of a light hardenable resin in the resin vat;    -   (b) positioning the build carrier and the light permeable base        at a predetermined distance relative to each other;    -   (c) emitting light by the light source toward the region between        the light permeable base and the build carrier for hardening the        hardenable resin in said region; and    -   (d) successively repeating steps (a) to (c),        whereby during building up the workpiece at least two different        light hardenable resins are supplied in the vat. In this        embodiment a portion of a light hardenable material may be        pre-provided in the resin vat, for example manually by a user or        automatically by one of the resin supplies.

The system is preferably configured such that each resin supply can becontrolled individually. Therefore, during building up the workpiece thedifferent resin supplies may be used to consecutively to providedifferent light hardenable resins in the vat. Although it is preferredthat the different light hardenable resins are provided on top of eachother in the vat, the system is preferably further configured to use twoor more resin supplies to simultaneously supply different lighthardenable resins in the vat. Thus, in case the different lighthardenable materials exhibit different colors and/or translucencies, aworkpiece, in particular a dental restoration, can be provided which hasa color gradation in one or more dimensions.

In one embodiment the system comprises a computer-based control system.The control system is preferably configured to control the resin supplydependent on the build progress. In particular, the control system maybe configured to initiate the supply of one of a plurality of differentlight hardenable resins in association with one or both of the steps (b)and/or (c).

In one embodiment the system is set up for performing the followingsteps:

-   -   providing a data based computer model of at least a portion of        the shape of the workpiece;    -   determining a volume of hardened resin or resin to be hardened        in at least one layer of the workpiece based on the computer        model; and    -   using the at least one resin supply to provide a quantity of a        light hardenable resin in the resin vat, wherein the quantity is        determined based on the determined volume of hardened resin or        resin to be hardened.

The system may further be set up to perform the step of virtuallyslicing the computer model into layers as a basis for layerwise buildingup the workpiece. The data based computer model is preferably arepresentation of part or all of the workpiece to be built up. Such acomputer model may be obtained from a computer aided design (CAD)system. The data may be in STL or any other appropriate data format.Further, the virtual slicing is preferably performed by a computeralgorithm which forms planar slices which peripheral boundarycorresponds to a portion of the outer boundary of the workpiece. Both,CAD techniques to provide data based computer models as well as slicingsoftware for converting the computer models into a format suitable inbuild-up processes, are known to the person skilled in the art.

Accordingly, the system of the invention preferably allows forusage-dependent supply of resin during building up the workpiece. Inparticular, the system of the invention is preferably configured toreplace resin that is hardened and retracted from a bath of resin in thevat by a corresponding quantity of new hardenable resin in a controlledmanner during building up the process. In this regard a “correspondingquantity” may include a compensation for tolerances that may occur inthe process. For example, unhardened resin that may adhere to hardenedportions of the workpiece and thus being retracted from the bath alongwith the workpiece may be accounted for in the step of supplying any newresin. Further, the system is preferably set up such that itautomatically selects the type of any hardenable resin supplied duringbuilding up the workpiece based on the computer model of the workpiece(or based on additional data associated with the computer model). Thus,the workpiece can be provided with different properties (for examplecolors and/or translucency) at different portions.

In one embodiment the resin supplies are arranged in a periphery of thelight permeable base and surrounding a build area within which theworkpiece can be built up. The resin supplies may extend into the vatand form a resin outlet adjacent the light permeable base. Further, theresin supplies may be provided within the vat. For example, the vat mayinwardly form resin outlets of associated resin supplies. Preferably vathas a circumferential vat wall within which the resin supplies may beprovided. Further, the resin supplies may be provided within the lightpermeable base. For example, the build surface may form resin outlets ofassociated resin supplies.

The system may be configured such that the different light hardenableresins can be supplied into the resin vat through the same resin supply.For example, the resin supply may be connectable to different storagetanks storing the different light hardenable resins.

In one embodiment the resin supplies each are in the form of a nozzle.Each nozzle may be selectively brought in fluid connection with one ormore resin storage tanks. Each nozzle preferably forms a free end of therespective resin supply. The nozzle may have a shape for guiding theresin in the vat in a particular way. For example the nozzle maycorrespond to a flat die for supplying the light hardenable resin in theform of a flat strand. This may help distributing the resin in the vatrapidly. Further, each resin supply may be connected or connectable toan associated storage tank holding a particular light hardenablematerial. Therefore, the system may have different storage tanks holdingdifferent light hardenable resins. The system may further have means tomerge different light hardenable resins. In particular, the resinsupplies may be connected or connectable to more than one storage tankeach holding a different light hardenable material.

In one embodiment the at least one resin supply is ring-shaped andsurrounds the build area. The resin supply may have a plurality ofnozzles distributed over the circumference. Alternatively, the resinsupply may have a flat nozzle extending circumferentially.

In a further embodiment the resin supplies and the build carrier arerotatable about a rotation axis that is arranged essentially parallel tothe build dimension. Further, the vat and the build carrier may berotatable about the rotation axis. This rotatability allows for mixingor agitating the hardenable resin in the vat. Further, the supply of thelight hardenable resin may be angularly adjusted relative to theworkpiece. Thus the color and color gradation of the workpiece may becontrolled.

In an embodiment the light source comprises an image projector. Aparticular image projector is based on Digital Light Processing™ (DLP).The image projector is preferably arranged for emitted light toward thelight permeable base. Further, the image projector is preferablyarranged for emitting light toward the build carrier, wherein the buildcarrier is arranged optically behind the light permeable base. The imageprojector is preferably configured to project the light in the form of atwo-dimensional pattern. The pattern is preferably in form of abitmap-based matrix. The resolution of the matrix is about 1920×1080pixels. The pattern therefore has bright pixels at which the lightimpinges on the light hardenable resin which is located on the lightpermeable base and is dark otherwise (so that outside the bright pixelsno light or an insignificant amount of light reaches the lighthardenable resin).

The light used in the system is selected such that it suits forhardening the light hardenable material. For example, the lighthardenable resin may comprise Irgacure® 819, a photo-initiator availablefrom BASF Germany, which absorbs light in a wavelength of about 310 nmto 430 nm. Other photo-initiators are possible as appropriate.Accordingly, the light used in the system of the invention comprises orconsists of light of this wavelength range. Other light sources may beused like for example LEDs (Light Emitting Diodes), Active-Matrix LEDs,Active-Matrix Organic LEDs or one or more lasers.

The light permeable base and the build carrier are preferably movablerelative to each other by computer control and thereby can be positionedrelative to each other. The system is preferably configured for movingthe light permeable base and the build carrier away from each otherstepwise in steps of between about 1 μm and 100 μm.

The system of the invention may comprise a light hardenable resin. Thelight hardenable resin may comprise a radically curable unsaturatedmonomer in an amount ranging from 20% to 99% per weight, an inorganicfiller in an amount ranging from 1% to 80% per weight and aphotoinitiator in an amount ranging from 0.001% to 5% per weight.

In a further aspect the invention relates to a method of additivemanufacturing a workpiece, in particular a dental workpiece. The methodcomprises the steps of:

-   -   (a) providing a system that comprises a resin vat forming a        light permeable base, a build carrier for holding a workpiece        built up by the system, and a light source which is arranged to        emit light through the light permeable base toward a region        between the light permeable base and the build carrier, the        system further comprising at least one resin supply;    -   (b) optionally using the at least one resin supply to provide a        portion of a light hardenable resin in the resin vat;    -   (c) positioning the build carrier and the light permeable base        at a predetermined distance relative to each other;    -   (d) emitting light by the light source toward the region between        the light permeable base and the build carrier for hardening the        hardenable resin in said region;    -   (e) successively repeating steps (b) to (d) for building up the        workpiece;        whereby during building up the workpiece step (a) is performed        at least a first time for supplying a first light hardenable        resin in the resin vat and a second time for supplying a        different second light hardenable resin in the resin vat.

During building up the workpiece at least two different light hardenableresins may be supplied in direct contact with each other in said vat.

In one embodiment the method comprises the steps of:

-   -   (a) providing a system that comprises a resin vat forming a        light permeable base, a build carrier for holding a workpiece        built up by the system, and a light source which is arranged to        emit light through the light permeable base toward a region        between the light permeable base and the build carrier, the        system further comprising at least one resin supply;    -   (b) using the at least one resin supply to provide a portion of        a light hardenable resin in the resin vat;    -   (c) positioning the build carrier and the light permeable base        at a predetermined distance relative to each other;    -   (d) emitting light by the light source toward the region between        the light permeable base and the build carrier for hardening the        hardenable resin in said region;    -   (e) successively repeating steps (b) to (d) for building up the        workpiece;        whereby at least two different light hardenable resins are used        in step (b).

The method preferably comprises the steps of providing a portion of alight hardenable resin in the resin vat, positioning the build carrierand the light permeable base at a predetermined initial distancerelative to each other and emitting light by the light source toward thelight hardenable resin between the light permeable base and the buildcarrier for hardening the hardenable resin. Thus, an initial layer ofhardened resin is generated that extends between the light permeablebase and the build carrier. For creating the initial layer the resin vatmay be pre-filled with a light hardenable resin. From this stage themethod may be further performed by step (b). Thereby the same or adifferent light hardenable resin may be used. Subsequently step (c) maybe performed with positioning the build carrier and the light permeablebase by a displacement of the build carrier over a distancecorresponding to the initial distance. Upon performing of step (d) andthereby exposing the resin between the initial layer and the lightpermeable base with light, a second layer is generated havingessentially the same thickness as the initial layer. From this stagesteps (b) to (d) may be repeated to build up the workpiece from aplurality of layers. Each new layer thus is created by emitting light tothe light hardenable resin between the light permeable base and the partof the workpiece already built. The predetermined distance preferablyincreases during building up the workpiece. The increase is controlledstepwise with essentially equal steps so as to layerwise build up theworkpiece. During building up a workpiece the light permeable base andthe build carrier are preferably moved away from each other stepwise.The steps are between about 200 μm and about 10 μm.

In one embodiment the method comprises the steps of:

providing a data based computer model of at least a portion of the shapeof the workpiece;

determining a volume of hardened resin or resin to be hardened in atleast one layer of the workpiece based on the computer model; and

using the at least one resin supply to provide a quantity of a lighthardenable resin in the resin vat, wherein the quantity is determinedbased on the determined volume of hardened resin or resin to behardened.

The method may further comprise the step of virtually slicing thecomputer model into layers as a basis for layerwise building up theworkpiece.

In one embodiment the method comprises one or more steps for building upan auxiliary compartment along with (in parallel to) building up theworkpiece. The auxiliary compartment preferably is shaped to separate apartial volume from a greater volume provided in the resin vat. Further,the auxiliary compartment is preferably formed by a membrane built fromthe light hardenable resin in the resin vat. The membrane may form afirst end which abuts the vat wall along a first line that extends froma first location on the vat wall adjacent the light permeable base to adifferent second location on the vat wall adjacent the light permeablebase. The membrane may further form a second end which extends parallelto the light permeable base along a second line that extends between thefirst and second location. The membrane preferably extends contiguouslybetween the first and the second line. Further, the membrane may belocated to enclose at least one resin outlet within the auxiliarycompartment. Therefore, the auxiliary compartment predominantly containsresin provided by the enclosed resin outlet. For example the auxiliarycompartment may contain a resin that has a different color than thecolor of the resin in the remainder of the vat. (Instead or in additionto different colors the resign may be adapted to assume differenttranslucencies, and/or mechanical properties like different hardnessesor different resiliences, for example.) Generally, the resins arepreferably chemically compatible. This enables building up a workpiecethat is not only shaded in the build dimension but that further hasdifferent color areas in a dimension lateral of the build dimension. Itis noted that the auxiliary compartment gets spaced from the lightpermeable base upon the step of positioning the build carrier and thelight permeable base at a predetermined distance relative to each other.However, although the auxiliary compartment in this situation does notfully seal with the light permeable base, different resins inside andoutside the auxiliary compartment remain substantially separate. This isbecause both, resin enclosed by the auxiliary compartment and resinpresent outside the auxiliary compartment, flow substantiallysimultaneously toward the light permeable base when the build carrierand the light permeable base are moved away from each other. The resinsfrom inside and outside the auxiliary compartment substantially meetunder the membrane along the second line. The viscosity of the resin maybe adjusted so that two different resins at the area at which they meetintermingle more or less partly. For example a low viscosity provideshigher intermingling than a high viscosity.

A CAD system may be provided for automatically generating such amembrane based on a shading designed in the object. The CAD system maybe configured to determine a merging line between two portions ofdifferent properties (for example differently colored portions) of theobject. The merging line may be used to automatically form a virtualprojection that correspond to the membrane.

The method preferably provides for usage-dependent supply of resinduring building up the workpiece. In particular, resin that is hardenedand retracted from a bath of resin in the vat may be replaced by acorresponding quantity of new hardenable resin in a controlled mannerduring building up the process. Further, the type of any hardenableresin supplied during building up the workpiece may be automaticallyselected based on the computer model of the workpiece (or based onadditional data associated with the computer model). Thus, the workpiececan be provided with different properties (for example colors and/ortranslucency) at different portions.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a partially cross-sectional front view of an additivemanufacturing system according to an embodiment of the invention;

FIG. 2 is a perspective view of an additive manufacturing systemaccording to an embodiment of the invention;

FIG. 3 is a perspective view of the system shown in FIG. 2 at adifferent stage of operation;

FIG. 4 is a partially cross-sectional front view of a further additivemanufacturing system according to an embodiment of the invention;

FIG. 5 is a perspective view of an additive manufacturing systemaccording to an embodiment of the invention;

FIG. 6 is a perspective view of the system shown in FIG. 5 at adifferent stage of operation;

FIG. 7 is a partially cross-sectional front view of an additivemanufacturing system according to an embodiment of the invention; and

FIG. 8 is a detail view of FIG. 7 .

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows an additive manufacturing system 1. The system 1 comprisesa light permeable base 2 which in the example is part of a vat 3. Thevat 3 and in particular the light permeable base 2 are made of atransparent material. Suitable materials include for example silicaglass or polycarbonate. Other materials are possible as appropriate. Thevat 3 is generally cup-shaped. In particular, the vat 3 has a bottomwall 3 a, a side wall 3 b and forms an opening 3 c opposite of thebottom wall 3 a. The vat 3 in the example is generally circular (has acircular side wall), although other geometries are possible.

The system 1 is generally configured to build up a workpiece 100 byadding increments or layers of material (hardened resin) at a bottom ofthe workpiece while successively moving the workpiece upwards. In thisregard the term “bottom” refers to an end of the workpiece whichessentially faces the center of gravity and the term “upwards” refers toa direction opposite of the center of gravity. Further, the workpiece100 is maintained submerged with its bottom in a bath of lighthardenable resin. Accordingly, the workpiece is successively pulled in adirection away from the resin bath while portions of material in thebath are hardened and thus added to the workpiece.

The system 1 comprises a build carrier 4. The build carrier 4 isconfigured to hold a workpiece built up by the system, as furtherexplained in detail below. Further the system 1 comprises a light source5, which in the example is an image projector that is based on DigitalLight Processing™. Digital Light Processing (DLP) uses micro-mirrorsarranged in a matrix on a semiconductor chip. Such semiconductor chipsare known as Digital Micromirror Devices (“DMD”). Typical mirrors of aDMD have a size of about 5 μm or less. Each of the mirrors are movablebetween two positions by control of the semiconductor. In the oneposition the mirror is positioned to reflect light directed on themirror through the light output, whereas in the other position themirror is positioned so that the light directed on the mirror does notexit the projector. Each mirror typically represents one pixel in theprojected image so that the number of mirrors typically corresponds tothe resolution of the projected image. The skilled person will recognizethat other projector techniques or a laser beam may be likewise usedwith the system of the invention.

In the example, the system has a housing 6 which has at least a lightpermeable area 7. The light permeably area is arranged generallyhorizontal (perpendicular to the direction of the force of gravity). Thevat 3 is removably placed with the light permeable base 2 on the lightpermeable area 7. Thus, light emitted by the light source 5 andtransmitted through the light permeable area 7 of the housing 6 is alsotransmitted through the light permeable base 2 of the vat 3. It ispreferred that the light permeable area 7 and the light permeable base 2are transparent and clear. Thus, the image sharpness of the imageprojected to the light permeable base is maximized. This is also a basisfor building up the workpiece at maximized accuracy.

The build carrier 4 is positionable relative to the light permeable base2 by computer control. In particular, the build carrier 4 is movable atleast in a dimension essentially perpendicular to the light permeablebase 2 (and perpendicular to the light permeable area 7). The dimensionessentially perpendicular to the light permeable base 2 is also referredto as “build dimension” herein (in the Figure designated as “B”).Further the build dimension is generally vertical (coinciding with thedirection of the force of gravity). The workpiece 100 is build up in thesystem 1 in the build dimension B. In particular the build-up process isperformed downwards (in the direction of the force of gravity) inrelation to a portion of the workpiece created first. This is achievedby pulling the workpiece successively upwards as it is built up.

In another example the build carrier may be movable in one or twofurther dimensions according to a three-dimensional Cartesian coordinatesystem. The build carrier 4 is connected to a linear drive 9 via asupport 8. The linear drive 9 in the example has a spindle (not shown)which is mechanically coupled to the support 8 such that the support 8can be moved in two directions in the build dimension B. The lineardrive 9 further has a motor 10 and a position measuring system. Thus,the support 8 and the attached build carrier 4 can be accuratelypositioned by control of the system 1. The skilled person will recognizethat the support can be directly configured as build carrier in anotherexample, and that the build carrier may be connected by other means withthe linear drive.

In the example, a dental crown forms the workpiece 100. At theillustrated stage the dental crown is already partially built up in thesystem 1. Generally, the workpiece 100 is build up in a region betweenthe build carrier 4 and the light permeable base 2. In particular, theworkpiece 100 is carried at a first end 101 of the workpiece 100 by thebuild carrier 4. The build carrier 4 at the stage shown is positionedsuch that a space 12 is formed between an opposite second end 102 of theworkpiece 100 and the light permeable base 2. The space 12 has apre-determined thickness in the build dimension. Further, a lighthardenable resin 11 is provided in the vat 3. The amount of the lighthardenable resin 11 is selected such that a resin bath of apre-determined fill level is form. The fill level of the lighthardenable resin 11 corresponds to or is higher than the thickness ofthe space 12. Accordingly, the space 12 is entirely filled with thelight hardenable resin 11. At this stage the image projector 5 can beused to emit light through the light permeable base 2 into the space 12.The light is preferably emitted in the form of a two-dimensional patternin a plane parallel to the light permeable base 2. Accordingly, thelight hardenable resin 11 is irradiated locally in accordance to thepattern of the light pattern. In particular, any light pixel of thepattern causes such portions of the light hardenable resin 11 to hardenwhich are exposed to the light of the light pixel. The light hardenableresin 11 is typically light permeable to a certain extent so that thelight penetrates entirely through the light hardenable resin 11 in thespace 12. Therefore, the hardened portions connect with the workpiece100 already built up and become part of a complemented workpiece. Fromthis stage the complemented workpiece can be retracted from the lightpermeable base 2 so as to create a new space filled with hardenableresin which can be irradiated by a further light pattern to furthercomplement the workpiece, and so on, until the workpiece is built uplayer by layer completely. The build carrier has a retention surface atwhich the workpiece 100 adheres. The retention surface provides for aretention of the hardened resin which is better than the retention ofthe hardened resin on the light permeable base 2. Thus, upon puling theworkpiece away from the light permeable base, the workpiece staysretained at the build carrier while it disconnects from the lightpermeable base. The skilled person will recognize several technicalpossibilities of controlling the better adhesion of the hardened resinat the build carrier, including the selection of materials for the buildcarrier 4 and the light permeable base 2, the configuration of thesurface roughness of the build carrier 4 and the light permeable base 2,the arrangement of retention elements, or a combination thereof. Thelight permeable base 2 is optionally coated with a non-stick coating,for example a polytetrafluorethylene. Thus, the hardened resin loosenseasily from the light permeable base while the hardened portions ofdifferent layers adhere to each other. Therefore, any breaking apart ofthe built up workpiece during retraction can be prevented.

During retraction of the workpiece hardenable resin in the vat is—bynature of physics—sucked (or pressed by the ambient pressure) in to theemerging space. To prevent the fill level from dropping below thethickness of the space (which could cause a void in the object) furtherhardenable resin is provided in the vat prior to and/or simultaneouslywith retraction of the workpiece.

The system 1 has a first and a second resin supply 13, 14. The firstresin supply 13 is provided for supplying a first light hardenable resinand the second resin supply is provided for supplying a different secondlight hardenable resin. The difference between the first and secondlight hardenable resin in the example is the color and/or translucency,but may in other examples be material strength, type, composition or acombination thereof. The first and second light hardenable resins arestored in first and second storage tanks 17, 18, respectively.

In a first operation mode of the system 1 the first hardenable resin isprovided by the first resin supply 13 in the vat 3 for building up oneor more layers of the workpiece and subsequently the second hardenableresin is provided by the second resin supply 14 in the vat 3 forbuilding up one or more further layers. At the stage of switching fromthe first to the second hardenable resin the second hardenable resin ispreferably provided onto a residual fill level of the first hardenableresin. Therefore, at the interface between the first and the secondhardenable resin the first and second hardenable resins merge and form asmooth transition. This allows for building up the workpiece with acolor gradation in which the colors change smoothly from one color tothe other. In particular dental restorations may be build up at a colorgradation which pleasantly resembles the color gradation of a naturaltooth. In the example the resin supply is controlled by the system 1based on computer instructions input in the system 1 via network,interface or a user.

In a second operation mode of the system 1 the first and second lighthardenable resins are simultaneously provided in the vat 3 by the firstand second resin supplies 13, 14 for building up one or more layers ofthe workpiece. The first and second hardenable resins may be provided indifferent amounts and/or timely offset. Accordingly, a single layer ofresin provided in the vat is formed of two different resins arrangedessentially side by side (in a dimension laterally to the builddimension). Therefore, the system 1 allows for building up workpieceshaving a color gradation in the build dimension, a color gradationlaterally thereto and a combination of both (for example inclined to thebuild dimension).

In the example, the vat 3 and the build carrier 4 may be rotatablerelative to each other. Further, the resin supplies 13, 14 and the buildcarrier 4 may be rotatable relative to each other. Therefore, byrotating the vat 3 and the resin supplies 13, 14 relative to each otherthe any resin in the vat 3 may be mixed by means of the resin supplies13, 14 extending into the resin. Further, by rotating the build carrier4 and the resin supplies 13, 14 relative to each other, different resinsas supplied by the different resin supplies 13, 14 may be arranged todifferent angular positions of the workpiece.

FIG. 2 shows a system 1 which is technically identical with the systemshown in FIG. 1 but which has two additional resin supplies 15, 16.Therefore, the system 1 has four resin supplies 13, 14, 15, 16 forsupplying four different light hardenable resins.

In FIG. 3 the system 1 is illustrated with an exemplary workpiece 100formed of different layers 100 a, 100 b, 100 c, 100 d of differenthardened resins. At the stage shown the workpiece 100 is fully retractedfrom the vat 3. The workpiece 100 as illustrated is obtained byconsecutively using the resin supplies 13, 14, 15, 16 to provide aportion of a light hardenable resin in the resin vat 3, positioning thebuild carrier 4 relative to the light permeable base 2, and emittinglight in the hardenable resin on the light permeable base 2. Thus, fourlayers of different hardened resins are formed.

FIG. 4 shows a system 1 which is technically identical with the systemshown in FIG. 1 , except for the arrangement of the resin supplies 13,14. In the example, the resin supplies 13, 14 are arranged in the sidewall 3 b. In particular, the resin supplies 13, 14 each form a resinoutlet in the vat side wall 3 b. In this example (although not shown),the vat 3 can be sized essentially according to the size of the buildcarrier 4 of the vat 3. Thus, the space in the vat may be usedrelatively efficiently. Further, the amount of resin not used forbuilding up the workpiece 100 may be minimized, for example byminimizing the vat size. In the system 1 more than the two resinsupplies 13, 14 may be provided. For example, a multiplicity of resinsupplies may be arranged uniformly distributed over the circumference ofthe vat wall. Thus, a multiplicity of different hardenable resins can beused for building up the workpiece 100. In this example the resinsupplies 13, 14 have a fixed position relative to the vat.

FIGS. 5 and 6 show a further example of the system 1 which istechnically identical with the system shown in FIGS. 2 and 3 except forthe arrangement of the resin supplies. In particular, resin supplies 13,14, 15, 16 are arranged essentially parallel to the build dimension Band each form an outlet adjacent the light permeable base 2. The resinsupplies 13, 14, 15, 16 extend through the build carrier 4. In theexample, the vat size can be minimized as described in the example ofFIG. 4 , but further a rotation of the vat 3 and the build carrier 4relative to each other can be used to mix any resin in the vat.

FIG. 7 shows a system 1 which is technically identical with the systemshown in FIG. 1 , except for the arrangement of the resin supplies 13,14. The first and second resin supply 13, 14 each form a first andsecond resin outlet 13 a, 14 a, respectively, in the vat side wall 3 b.Although the example may differentiate from other examples by thearrangement of the resin supplies 13, 14 the build-up process describedin the following can be performed with any of the systems described (inparticular but not exclusively in the FIGS. 1-6 ) herein.

In the example, the system is set up so that the first resin supply 13provides a first resin and so that the second resin supply 14 provides asecond resin. The first and second resin are different, in particularhave different colors in the example.

At the stage as illustrated a workpiece 100 has been at least partiallybuilt up. Further, an auxiliary compartment 200 has been built uptogether with the workpiece 100. The auxiliary compartment 200 is formedby a (preferably thin) membrane. The membrane is built from the lighthardenable resin in the resin vat 3. Because the membrane is preferablythin the auxiliary compartment 200 can be removed from the workpiece 100without substantially damaging the workpiece 100.

The auxiliary compartment is shaped to separate a partial volume A froma greater volume B provided in the resin vat 3.

The workpiece 100 and the auxiliary compartment 200 are shown in moredetail in FIG. 8 . The membrane forms a first end 201 which abuts thevat wall (not illustrated in this view) along a first line that extendsfrom a first location 202 on the vat wall adjacent the light permeablebase (not illustrated in this view) to a different second location 203on the vat wall adjacent the light permeable base. The membrane furtherforms a second end 204 which extends parallel to the light permeablebase along a second line that extends between the first and secondlocation 202, 203. The membrane extends contiguously (as a closedstructure) between the first and the second line 203. Because the firstand second line each extend essentially U-shaped the membrane in essencecorresponds in shape to a section of an outer shell of a donut.

Further, the membrane encloses the second outlet 14 a within theauxiliary compartment 200. Accordingly, resin provided through thesecond resin supply 14 flows directly into the auxiliary compartment. Onthe other hand, resin provided through the first resin supply 13 flowsdirectly into the resin vat outside the auxiliary compartment 200.Therefore, the auxiliary compartment 200 predominantly contains resinprovided by the second resin supply 14, and the remainder of the resinvat 3 predominantly contains resin provided by the first resin supply13. It is noted that more than one auxiliary compartment may be providedeach enclosing one or more resin supplies. Further one auxiliarycompartment enclosing several resin supplies may be provided. More thanone resin supplies may further be provided for the remainder of theresin vat for providing resins in the resin vat outside of the auxiliarycompartment(s).

What is claimed is:
 1. A method of additive manufacturing a workpiece,comprising the steps of: (a) providing a system that comprises a lightpermeable base, a build carrier for holding a workpiece built up by thesystem, and a light source which is arranged to emit light through thelight permeable base toward a region between the light permeable baseand the build carrier, the system further comprising a resin vat and atleast one resin supply, wherein the light permeable base forms a wallportion of the resin vat, (b) using the at least one resin supply toprovide a portion of a light hardenable resin in the resin vat; (c)positioning the build carrier and the light permeable base at apredetermined distance relative to each other; (d) emitting light by thelight source toward the region between the light permeable base and thebuild carrier for hardening the hardenable resin in said region; (e)successively repeating steps (b) to (d) for building up the workpiece;and (f) building up an auxiliary compartment along with building up theworkpiece, wherein the auxiliary compartment is shaped to separate apartial volume from a greater volume provided in the resin vat, wherebyduring building up the workpiece step (a) is performed at least a firsttime for supplying a first light hardenable resin in the resin vat and asecond time for supplying a different second light hardenable resin inthe resin vat.
 2. The method of claim 1, wherein the predetermineddistance increases during building up the workpiece and wherein theincrease is controlled stepwise with essentially equal steps so as tolayerwise build up the workpiece.
 3. The method of claim 2, comprisingthe steps of providing a data based computer model of at least a portionof the shape of the workpiece; determining a volume of hardened resin orresin to be hardened in at least one layer of the workpiece based on thecomputer model; and using the at least one resin supply to provide aquantity of a light hardenable resin in the resin vat, wherein thequantity is determined based on the determined volume of hardened resinor resin to be hardened.
 4. The method of claim 1, wherein the auxiliarycompartment is formed by a membrane built from the light hardenableresin in the resin vat.
 5. The method of claim 1, whereby successivelyrepeating steps (b) to (d) for building up the workpiece comprises, atleast once, supplying the first or second light hardenable resindirectly to the auxiliary compartment.